1. FIELD OF THE INVENTION
The present invention relates to a method of producing an infrared luminescent diode using GaAs, of the so-called planar type in which a portion or the whole of the edge of the PN junction is exposed in one of the principal surfaces of the substrate.
2. DESCRIPTION OF THE PRIOR ART
Infrared luminescent diodes (hereafter referred to as GaAs LED) using silicon-doped (hereafter referred to as "Si-dopped") gallium arsenide are widely applied in such devices as photocouplers and display elements using the conversion of infrared rays to visible light since they have various advantages: low operating voltages of 1-2 volts, high luminescent efficiency of 3-5%, small size with long life, and facility in fabrication with a PN junction formed by a single process of liquid growth. The Si-doped GaAs LED of this conventional type has been fabricated by forming a Si-doped N-type GaAs layer having a thickness of 5-20 .mu. on one of the principal surface of an N-type GaAs substrate and further by forming on the N-type GaAs layer a Si-doped P-type GaAs layer having a thickness of 50-100 .mu., through the liquid phase growth techniques. FIG. 1 shows a GaAs LED fabricated according to this method. In FIG. 1 reference numeral 1 indicates an N-type GaAs substrate, 2 an N-type GaAs layer, 3 a P-type GaAs layer, and 4 and 5 a pair of main electrodes in ohmic contact respectively with the substrate 1 and the layer 3. With this structure in which the main electrodes are attached on the opposite surfaces, the connection of these electrodes with other terminals cannot resort to the face-down bonding and the connection of at least one of the electrodes must be performed through wire bonding or the beam lead bonding techniques. Accordingly, the fabrication of such a GaAs LED is rather laborious and has a poor mass-producibility, the mechanical strength and the reliability of the thus formed GaAs LED are insufficient, and the connection of the GaAs LED with the light receiving element of photocouplers becomes complicated. In order to eliminate these drawbacks, the selective diffusion, the ion implantation or the epitaxial growth techniques for rendering the GaAs LED in the planar configuration, have been proposed. However, since the GaAs LED obtained through the diffusion and/or the ion implantation usually has a Zn-doped P-type layer, it has a lower luminescent efficiency than the Si-doped GaAs LED. On the other hand, according to the selective epitaxial growth technique, in which a recess is formed in the surface of a GaAs substrate with an SiO.sub.2 film used as mask and the liquid-phase growth takes place in the recess, a smooth and uniform layer can be obtained only with great difficulty because of the following drawbacks. Namely, due to the anisotropy in the growth velocity and to the unevenness in the supply of the melt or raw material, (1) the GaAs layer 7 extends even over the mask 6, (2) the edge portion of the recess 10 grows rapidly, as shown in FIG. 2, and (3) under certain conditions of epitaxial growth the mouth of the recess is occluded to leave in the recess the melt, which gives rise to a polycrystalline region.